1. Technical Field
The present invention relates to computer memory storage systems and more particularly to a system for managing archival and retrieval tasks for an existing host computer.
2. Discussion
The memory capacity of a computer system is limited by a number of factors. Paramount among these factors is the access time required for the computer to retrieve and make use of stored data. In general, the larger the storage capacity and the more data there is stored, the longer it takes to access this data. Conversely, if only a relatively small storage capacity is required, systems can access this limited data base relatively quickly. Thus, most computer systems represent a compromise between quick access to a small capacity memory at one extreme and slow access to a large capacity memory at the other.
For example, hard disk storage is a relatively quick access but small capacity storage medium. A typical single hard disk can store 200 M bytes of memory and can be accessed in less than one second. Hard disk storage capacity can be increased by adding additional hard disks and drives, however, as the number of disks increases the system throughput will decrease due to the additional time required for the computer to search through all of the disks when attempting to locate a given file.
One alternative is to store infrequently used files on paper, magnetic tape or microfiche. These offer an unlimited amount of archival data storage. However, these methods can be costly and time consuming. First, the physical space required to store information on paper, microfiche or computer tape can be costly. Also, there are significant transfer costs in transferring the information from electronic to the physical media and back again. For example, microfiche and printouts must be reentered to the system either by manually keying in the information, or by utilizing optical character readers. In addition, archival information stored using these methods must first be located manually. This raises the access time significantly. In many cases, the process of requesting, locating and making a file available stored in one of these three methods, can as a practical matter, raise the access times to a matter of days. With microfiche additional problems are often experienced such as the cost of microfiche viewing machines and delays in access to such machines.
Also, hard disk systems are relatively expensive; and as they become larger they become more of a burden on the host to manage. Storage life is another important consideration and with magnetic tape a typical lifetime is six to eight years.
An attractive alternative because of these problems is optical disk storage. An individual optical disk can store up to 1,800 megabytes of compressed data with a storage life of over 15 years. Since the information is written permanently and cannot be erased, these "write-once" disks are among the most permanent form of data storage available. Also, optical disks offer the lowest storage cost per megabyte. Access time is relatively good for optical disks. For example, in a 20 gigabyte optical disk storage system a file can typically be accessed in less than one minute with the optical disk system connected directly to the system bus of a host computer.
While optical disk storage offers a viable replacement for tape libraries and microfilm, it is not without disadvantages. First, the addition of an optical storage system typically requires modifications to the software and/or hardware of the host computer. Further, in the typical configuration with the optical disk connected directly to the system bus of the host, the host CPU can quickly become overloaded in performing tasks associated with managing the transfer of data to and from the optical disk. These tasks include such functions as controlling which transactions are sent out to the archive and when they are sent; and housekeeping functions such as compressing data, and determining where on an optical disk a file is to be stored.
For these reasons, optical disk storage can impose a significant burden on the CPU. Thus, the full potential of optical disk as a data storage medium for computer systems has not yet been realized due to problem in getting data into and out of the host computer as well as managing these processes.
Thus, it would be desirable to provide a system which cost effectively extends the on-line storage capabilities of computer systems. It would also be desirable to provide such a system which has relatively large storage capacity and at the same time which can be accessed in a relatively short amount of time. It would additionally be desirable to provide an archival data storage system which effectively can replace tape libraries and microfilm. Further, it would be desirable to provide an archival data storage system, which can utilize optical data storage media and which can be coupled to a computer system without requiring hardware or software modifications to the host system. Also it would be desirable to provide a system which minimizes the work needed to be accomplished by the host in managing data archival and retrieval functions. In other words, it would be desirable to provide a system which utilizes optical storage media without requiring significant host CPU time. Further, it would be desirable to provide a system with the above features at a moderate cost and which utilizes existing hardware platforms.